An interconnect circuit board of an LTCC design is a physical realization of electronic circuits or subsystems made from a number of extremely small circuit elements that are electrically and mechanically interconnected. It is frequently desirable to combine these diverse electronic components in an arrangement so that they can be physically isolated and mounted adjacent to one another in a single compact package and electrically connected to each other and/or to common connections extending from the package.
Complex electronic circuits generally require that the circuit be constructed of several layers of conductors separated by insulating dielectric layers. The conductive layers are interconnected between levels by electrically conductive pathways, called vias, through a dielectric layer. Such a multilayer structure allows a circuit to be more compact than traditional Al2O3 substrates by allowing vertical integration.
LTCC tape has been widely used in the automotive and telecom industry for its multilayer, cofiring and flexible design capabilities. One of the critical elements in the successful use of the materials is that the surface conductors need to have good soldered adhesion under both thermal aging (isothermal storage at 150° C.) and thermal cycling (typically between a low temperature in the range of −55 to −40° C. and a high temperature in the range of 100-150° C.) conditions. Such exposure often will result in the development of stresses in the solder joints used to attach surface mounted components to the substrate. The primary reason for these stresses is a mismatch in thermal expansion of the various different materials that comprise the joint, namely, the ceramic, the conductor metal, the solder metal, the metal that comprises the leads to the surface-mounted device and the material used to make the device. Through careful layout design and skilled application of underfill materials one can distribute the stresses more evenly and prevent a concentration of stresses at any one joint. However, it is not possible to eliminate these stresses completely. One key to sustaining good soldered adhesion during such exposure is the ability of the materials involved to absorb these stresses without undergoing any permanent or irreversible mechanical damage. In other words any improvement in the mechanical properties, i.e., elastic and bulk modulus, of the materials, particularly those of the underlying tape will increase the ability of the overall joint to absorb stress during thermal exposure.
One of the most common failure modes observed in the thermal cycling of solder joints is cracking of the tape at the periphery of the conductor pad. As exposure continues such cracks can propagate through the dielectric and under the pad itself. In some cases the onset of cracking has been observed after less than 10 cycles. This is compared to a typical specification which requires 500 thermal cycles without evidence of ceramic tape cracking or significant loss of adhesion between the conductor and the underlying tape.
U.S. Pat. No. 5,431,718 to Lombard et al. provides a high-adhesion strength, co-fireable, solderable silver metallization material for use with low-fire ceramics. The metallization material includes the metal powder as well as an organic vehicle, and adhesion promoting agents. This combination of elements allows a metallization material which can be cofired at relatively low temperatures necessary for firing ceramic substrate materials while providing an adequate base for soldering subsequent circuit components to the ceramic substrate.
U.S. Pat. No. 4,416,932 to Nair discloses a ceramic substrate having a conductive pattern coating wherein the coating comprises an admixture of finely divided particles of a noble metal or alloy, a low melting, low viscosity glass, a spinel-forming metal oxide and an organo-titanate and the process of making same.
Neither of the above inventions meets the specification of 500 cycles without cracking of the dielectric and maintaining adhesion of the conductor to the substrate during thermal exposure. Accordingly, there exists a need for a conductor composition which is able to surpass the thermal cycling capabilities of the prior art. In particular, there is a need for a conductor composition which can exceed 500 thermal cycles without cracking or losing adhesion on the tape substrate. The present invention provides such conductor compositions.